Mattresses of the aforementioned type generally comprise a spring casing in which a number of spring units, generally helical in shape, are suitably connected to each other while being disposed between to imaginary planes parallel to the bases of the mattress. More specifically, these spring units are coplanar on their ends, where they are attached to padded bodies that line the core of the mattress on its top and bottom, softening the physical contact with said springs and thereby increasing the comfort of the mattress for the user.
A similar padded lining is present around the entire perimeter of the mattress, where the result is less effective as the deformability of these areas is greater, particularly so in the corners, where bulges, recesses, deformations or wrinkles often appear that harm the appearance of the mattress.